Tamper-evident induction foil lining systems have been used in the container closure industry for more than twenty years. Usage of such lining systems has increased significantly within the past decade, because of tampering with the contents of over-the-counter drugs and other products.
When conventional induction foil disks are used to seal the closure opening, removal of the disks is frequently frustrating and difficult. When the disk is fused to the container opening, removal or opening of the foil inner seal usually requires the use of a sharp object or fingernail to break, with considerable force, the foil; so that it can be torn away from the opening. The benefits of providing a tamper-evident seal across the container opening are offset by the considerable difficulty in removing the seal from the opening, and the resulting consumer dissatisfaction.
Recently, some induction foil disk systems have been provided with an integral pull-tab to facilitate removal of the foil disk from the container opening. The tab is intended to be gripped by the consumer, and pulled upward and across the container opening to peel a foil polymer disk off the container lip. This approach has utilized a "peelable bond" induction foil material to seal the container opening. When a pull-tab is used with such a "peelable bond" material, the liner theoretically is removed with a clean peel off the container opening. The amount of force to achieve this, theoretically at least, is less than that required to break the aluminum foil layer when the liner is fused to the lip of the container.
Several problems exist in conjunction with peelable bond systems, however. First of all, for materials presently used in such systems, it is difficult for the packager to achieve a hermetic seal at all points on the container lip (particularly beneath the folded tab, which can shield the area beneath it from the induction sealing energy field), while also maintaining a "peelable" bond strength at which the tab can function. The processing window for producing peelable bond sealing of the foil liner to the container opening is relatively narrow (that is, a narrow range of processing heat and pressure will provide the desired "peelable" bond strength). In contrast, the processing window to create a "fusion bond" seal is relatively wide. The result is that many packagers who want to use a pull-tab system cannot, due to the narrow processing window for peelable bond systems, and limited process control capabilities. These limited capabilities result from poor application torque control, unskilled work force, equipment limitations, short-run productions, and others.
Even if all of the processing limitations can be met to provide a good "peelable bond" induction foil seal, a lower degree of tamper evidency results. This is because the inherent nature of a "peelable" bond is to leave a clean bottle lip surface when the liner is removed from the container opening. This lessened tamper evidency is considered objectionable by much of the vitamin, over-the-counter drug and pharmaceutical industry, and has limited interest of many packagers who require readily visible tamper evidency when the liner is removed. Fusion bonded materials provide such a high level of tamper evidency because a polymer or foil residue remains on the container lip after the liner rupture or removal has taken place.
Another problem with peelable bond pull-tab systems is that the proper operation of a peelable bond integral pull-tab liner is totally dependent upon the relative strength of the liner/bottle-opening bond strength versus the tensile strength or tear resistance of the tab itself. Unfortunately, the bond strength of the liner to the container opening varies considerably with processing conditions. Consequently, it has been a common occurrence for induction sealing process variations to produce bonds which are too strong for the tab to handle. If the bond strength is too high, the tab snaps or tears without removing the liner disk from the container opening. This leaves the consumer frustrated with the performance of the tab system.
Consequently, it is desirable to provide a pull-tab tamper-evident induction foil lining system which overcomes the disadvantages noted above for the prior art systems.